Cyclin-dependent kinases (CDKs) are important cellular enzymes that perform essential functions in regulating eukaryotic cell division and proliferation. The cyclin-dependent kinase catalytic units are activated by regulatory subunits known as cyclins. At least sixteen mammalian cyclins have been identified (Johnson D G, Walker C L. Cyclins and Cell Cycle Checkpoints. Annu. Rev. Pharmacol. Toxicol. (1999) 39:295-312). Cyclin B/CDK1, cyclin A/CDK2, cyclin E/CDK2, cyclin D/CDK4, cyclin D/CDK6, and likely other heterodynes are important regulators of cell cycle progression. Additional functions of cyclin/CDK heterodynes include regulation of transcription, DNA repair, differentiation and apoptosis (Morgan D O. Cyclin-dependent kinases: engines, clocks, and microprocessors. Annu. Rev. Cell. Dev. Biol. (1997) 13:261-291). Cyclin-dependent kinase inhibitors have been demonstrated to be useful in treating cancer. Increased activity or temporally abnormal activation of cyclin-dependent kinases has been shown to result in the development of human tumors, and human tumor development is commonly associated with alterations in either the CDK proteins themselves or their regulators (Cordon-Cardo C. Mutations of cell cycle regulators: biological and clinical implications for human neoplasia. Am. J. Pathol. (1995) 147:545-560; Karp J E, Broder S. Molecular foundations of cancer: new targets for intervention. Nat. Med. (1995) 1:309-320; Hall M, Peters G. Genetic alterations of cyclins, cyclin-dependent kinases, and Cdk inhibitors in human cancer. Adv. Cancer Res. (1996) 68:67-108). Amplifications of the regulatory subunits of CDKs and cyclins, and mutation, gene deletion, or transcriptional silencing of endogenous CDK inhibitors have also been reported (Smalley et al. Identification of a novel subgroup of melanomas with KIT/cyclin-dependent kinase-4 overexpression. Cancer Res (2008) 68: 5743-52).
Clinical trials for the CDK4/6 inhibitors palbociclib, ribociclib and abemaciclib are ongoing for breast and other cancers, as single agents or in combination with other therapeutics. Palbociclib and ribociclib have been approved for treatment of hormone receptor (HR)-positive, human epidermal growth factor receptor 2 (HER2)-negative advanced or metastatic breast cancer in combination with aromatase inhibitors in post-menopausal women, and for palbociclib, in combination with fulvestrant after disease progression following endocrine therapy, (O'Leary et al. Treating cancer with selective CDK4/6 inhbitors. Nature Reviews (2016) 13:417-430). While CDK4/6 inhibitors have shown significant clinical efficacy in ER-positive metastatic breast cancer, as with other kinases their effects may be limited over time by the development of primary or acquired resistance.
Overexpression of CDK2 is associated with abnormal regulation of cell-cycle. The cyclin E/CDK2 complex plays and important role in regulation of the G1/S transition, histone biosynthesis and centrosome duplication. Progressive phosphorylation of Rb by cyclin D/Cdk4/6 and cyclin E/Cdk2 releases the G1 transcription factor, E2F, and promotes S-phase entry. Activation of cyclin A/CDK2 during early S-phase promotes phosphorylation of endogenous substrates that permit DNA replication and inactivation of E2F, for S-phase completion. (Asghar et al. The history and future of targeting cyclin-dependent kinases in cancer therapy, Nat. Rev. Drug. Discov. 2015; 14(2): 130-146).
Cyclin E, the regulatory cyclin for CDK2, is frequently overexpressed in cancer. Cyclin E amplification or overexpression has long been associated with poor outcomes in breast cancer. (Keyomarsi et al., Cyclin E and survival in patients with breast cancer. N Engl J Med. (2002) 347:1566-75). Cyclin E2 (CCNE2) overexpression is associated with endocrine resistance in breast cancer cells and CDK2 inhibition has been reported to restore sensitivity to tamoxifen or CDK4 inhibitors in tamoxifen-resistant and CCNE2 overexpressing cells. (Caldon et al., Cyclin E2 overexpression is associated with endocrine resistance but not insensitivity to CDK2 inhibition in human breast cancer cells. Mol Cancer Ther. (2012) 11:1488-99; Herrera-Abreu et al., Early Adaptation and Acquired Resistance to CDK4/6 Inhibition in Estrogen Receptor-Positive Breast Cancer, Cancer Res. (2016) 76: 2301-2313). Cyclin E amplification also reportedly contributes to trastuzumab resistance in HER2+ breast cancer. (Scaltriti et al. Cyclin E amplification/overexpression is a mechanism of trastuzumab resistance in HER2+ breast cancer patients, Proc Natl Acad Sci. (2011) 108: 3761-6). Cyclin E overexpression has also been reported to play a role in basal-like and triple negative breast cancer (TNBC), as well as inflammatory breast cancer. (Elsawaf & Sinn, Triple Negative Breast Cancer: Clinical and Histological Correlations, Breast Care (2011) 6:273-278; Alexander et al., Cyclin E overexpression as a biomarker for combination treatment strategies in inflammatory breast cancer, Oncotarget (2017) 8: 14897-14911.)
Amplification or overexpression of cyclin E1 (CCNE1) is also associated with poor outcomes in ovarian, gastric, endometrial and other cancers. (Nakayama et al., Gene amplification CCNE1 is related to poor survival and potential therapeutic target in ovarian cancer, Cancer (2010) 116: 2621-34; Etemadmoghadam et al., Resistance to CDK2 Inhibitors Is Associated with Selection of Polyploid Cells in CCNE1-Amplified Ovarian Cancer, Clin Cancer Res (2013) 19: 5960-71; Au-Yeung et al., Selective Targeting of Cyclin E1-Amplified High-Grade Serous Ovarian Cancer by Cyclin-Dependent Kinase 2 and AKT Inhibition, Clin. Cancer Res. (2017) 23:1862-1874; Ayhan et al., CCNE1 copy-number gain and overexpression identify ovarian clear cell carcinoma with a poor prognosis, Modern Pathology (2017) 30: 297-303; Ooi et al., Gene amplification of CCNE1, CCND1, and CDK6 in gastric cancers detected by multiplex ligation-dependent probe amplification and fluorescence in situ hybridization, Hum Pathol. (2017) 61: 58-67; Noske et al., Detection of CCNE1/URI (19q12) amplification by in situ hybridisation is common in high grade and type II endometrial cancer, Oncotarget (2017) 8: 14794-14805).
The small molecule inhibitor, dinaciclib (MK-7965) inhibits CDK1, CDK2, CDK5 and CDK9 and is currently in clinical development for breast and hematological cancers. Seliciclib (roscovitine or CYC202), which inhibits CDK2, CDK7 and CDK9, is being investigated for treatment of advanced solid tumors in conjunction with chemotherapy. Despite significant efforts, there are no approved agents targeting CDK2 to date. Cicenas et al. Highlights of the Latest Advances in Research on CDK Inhibitors. Cancers, (2014) 6:2224-2242. There remains a need to discover CDK inhibitors having novel activity profiles, in particular those targeting CDK2.